Infrared Studies of Molecular Shocks in the Supernova Remnant HB 21: II. Thermal Admixture of Shocked H$_2$ Gas in the South

TL;DR

This study investigates molecular shocks in the supernova remnant HB 21's southern region using infrared observations, modeling H2 gas emission with a thermal admixture approach to understand shock-cloud interactions.

Contribution

It introduces a thermal admixture model for H2 gas emission in supernova remnant shocks and compares different shock interaction scenarios, expanding understanding of shock-cloud dynamics.

Findings

H2 gas follows a power-law temperature distribution with specific parameters.

Observed H2 emission exceeds model predictions by a factor of four.

Analysis of shock models reveals strengths and weaknesses in explaining observations.

Abstract

We present near- and mid-infrared observations on the shock-cloud interaction region in the southern part of the supernova remnant HB 21, performed with the InfraRed Camera (IRC) aboard AKARI satellite and the Wide InfraRed Camera (WIRC) at the Palomar 5 m telescope. The IRC 4 um (N4), 7 um (S7), and 11 um (S11) band images and the WIRC H2 v=1->0 S(1) 2.12 um image show similar diffuse features, around a shocked CO cloud. We analyzed the emission through comparison with the H2 line emission of several shock models. The IRC colors are well explained by the thermal admixture model of H2 gas--whose infinitesimal H2 column density has a power-law relation with the temperature $T$, $dN\sim T^{-b}dT$--with n(H2) $\sim3.9\times10^4$ cm^{-2}, $b\sim4.2$, and N(H2;T>100K) $\sim2.8\times10^{21}$ cm^{-2}. We interpreted these parameters with several different pictures of the shock-cloud interactions--multiple planar C-shocks, bow shocks, and shocked clumps--and discuss their weaknesses and strengths. The observed H2 v=1->0 S(1) intensity is four times greater than the prediction from the power-law admixture model, the same tendency as found in the northern part of HB 21 (Paper I). We also explored the limitation of the thermal admixture model with respect to the derived model parameters.